The crystal structure of β-CsB₅O₈ has been determined from X-ray powder diffraction data using synchrotron radiation: Pbca, a=7.8131(3) Å, b=12.0652(4) Å, c=14.9582(4) Å, Z=8, ρ_calc = 2.967 g/cm³, R-p=0.076, R-wp=0.094. β-CsB₅O₈ was found to be isostructural with β-KB₅O₈ and P-RbB₅O₈. The crystal structure consists of a double interlocking framework built up from B-O pentaborate groups. The crystal structure exhibits a highly anisotropic thermal expansion: α_a=53, α_b=16, α_c= 14.10^-6/K; the anisotropy may be caused by partial straightening of the screw chains of the pentaborate groups. The similarity of the thermal and compositional (Cs-Rb-K substitution) deformations CsB₅O₈ is revealed: increasing the radius of the metal by 0.01 Å leads to the same deformations of the crystal structure as increasing the temperature by 35°C.